1. Field of the Invention
The present invention relates generally to rotors and electrical machines using rotors, and, more particularly, but not by way of limitation, to permanent-magnet (PM) rotors, rotor components, and machines comprising PM rotors.
2. Description of Related Art
Rotors and machines using rotors have been known in the art for some time. For example, electrical machines such as motors traditionally include brushes and a rotor with wire windings. Such traditional winding-based rotors are relatively inefficient compared with PM rotors, and PM rotors may generally be made smaller than traditional winding-based rotors for a given capacity or output. As a result, machines using PM rotors may generally be made smaller than those with traditional winding-based rotors. PM rotors generally use permanent magnet material instead of traditional electrical wire windings, such that a PM rotor machine does not require reactive current from a power supply. As such, power consumption of PM machines can generally be reduced relative to those using traditional winding-based rotors for a given output. For example, some PM rotor machines can achieve a higher power factor, higher power density, and higher efficiency (e.g., 25% to 120% of the rated load), relative to a traditional winding-based machine. Maintenance costs of PM machines may also be reduced, as the simpler configuration (e.g., without windings, brushes, etc.) may result in fewer parts or points of potential failure.
PM electric motors, which are widely used at present in daily life, industrial and agricultural production, aviation and aeronautics, national defense, and various like fields, are generally of one of the following types: PM synchronous generators, PM DC motors, PM motors with asynchronous start-up, variable speed PM synchronous motors, and other PM motors having special uses. Variable speed PM synchronous motors can generally start and stop fast, operate at variable speeds, and perform high-speed tracking under relatively easy modes of control. Moreover, through appropriate design of such motors, the harmonic torque generated by the interaction of stator current and rotor magnets, and the cogging torque generated by the interaction of the stator core and rotor magnets, can be reduced, allowing the motor when operating at low speed to maintain a low speed ripple and high positional accuracy. Hence these motors are widely used in the field of industrial-control servos.
Variable speed PM synchronous motor may be divided into two general types: square-wave PM synchronous motors (also known as the “brushless DC motor” or “BDCM”) and sine-wave PM synchronous motors or “PMSM”. With respect to motor structure, the stator armature winding of BDCMs is generally a 60-degree phase-belt concentrated full-pitch winding. Stator winding of PMSMs is generally similar to an ordinary Y-series asynchronous motor, for which distributed winding may be used, and can be full-pitch or short-pitch. In both types of motor, permanent magnets or interior permanent magnets are installed on the rotor. In contrast to asynchronous electric motors, stator current flux can be reduced by the presence of the permanent magnets, and stator copper loss can thereby also be reduced. There is generally no copper loss on the rotors because the sides of the rotors are free of the aluminum bars which distinguish squirrel-cage inductance motors, and hence efficiency is generally improved. Heat generated by the motor may also be reduced, which can be very important for high-precision servo systems. In addition, variable speed PM synchronous motors generally have greater power density, wider speed-adjustment range, and larger torque/current ratio than typical asynchronous electric motors. One field of use is in servo applications, in which electric motors with compact structure, small size, and broad speed-adjustment ranges are sought.
When a variable speed PM synchronous motor is used as an actuator in a digitally controlled servo system, its rotational speed pulsation is generally also required to be as small as possible; that is, the motor's pulsating torque is required to be as small as possible. The pulsating torque of a variable speed PM synchronous motor primarily refers to cogging torque and torque ripple. In terms of characteristics, there is often little or no difference between the cogging torque of a BDCM and that of a PMSM; both are generally pulsating torques that may be generated as a result of the presence of stator teeth, and both typically have the character of reluctance torque that may be the result of the effects of permanent magnets on the stator core. This may be related to rotor position and often changes as the rotor rotates; e.g., it is a spatial function of an electric motor. Methods for weakening and eliminating cogging torque are often similar for both kinds of motors: for example, using skewed stator slots or skewed rotor poles or choosing an appropriate pole-arc coefficient. The goal is usually to keep the apparent magnetic resistance of the rotor constant from beginning to end, even when the rotor is rotating. In general, by skewing a stator slot or a permanent magnet by the distance of one tooth, it may be possible to reduce cogging torque to less than 2% of its set value. Thus, there is generally no significant difference between the cogging torques of BDCM's and PMSM's.
Torque ripple is somewhat unrelated to cogging torque inasmuch as the two are generated by different causes. Torque ripple is typically a result of the interaction between stator current and flux in the magnetic field of the permanent magnets. Both for a BDCM and for a PMSM, whenever the stator current deviates from the ideal wave form, torque ripple may be caused. By means of current control, it is generally possible to ensure that the current feeding a PMSM is sinusoidal, but it is often impossible to ensure that the current feeding a BDCM is square-wave current. This is so because the electrical inductance of a BDCM generally prevents stator phase current from undertaking sudden fluctuations, with the result that the current's actual wave-form is generally trapezoidal. These deviations in current wave-form can cause BDCM torque to exhibit large phase-change ripples, while in PMSM's these ripples are usually fairly small. In high-speed operation, it is possible for these torque ripples to be filtered out by rotor inertia. But in low-speed operation, these torque ripples can negatively affect servo system performance. Thus, BDCM's can be used in speed servo systems and position servo systems that do not have very high performance requirements, but PMSM's may function better in high-performance speed servo systems and position servo systems. In addition to the foregoing characteristics, PMSM's have gained popularity and functionality with the development of electrical and electronic technology, microelectronics, and computer technology, especially since the popularization in the electric motor speed-control field of the concept of using vector control for alternating current motors (which was proposed by German scholars Blaschke and Hases in the 1970's). Nevertheless, PMSM's often have better dynamic properties and better tracking properties, and PMSM's are widely used in high-precision servo systems.
In some known PM rotor configurations, PM bulks are internally coupled to the rotor (e.g., are coupled to the rotor within the external perimeter of the rotor core. One example of such a PM rotor may be referred to as a radial-magnet rotor configuration, in which the magnetizing direction of the PM bulks is aligned with the radial direction of the rotor (e.g., along a line extending from the axis of rotation outward). The challenge for this rotor configuration is that the magnetic-insulation bridges between adjacent PM bulks must be large enough to avoid mechanical damage (e.g., resist mechanical stresses) during high-speed rotation of the rotor. But if the magnetic insulation bridge is large, the magnetic insulation provided between two poles (N and S poles) may be insufficient to effectively limit flux leakage between the poles. This flux leakage may lead to reduced performance of the rotor such that larger PM bulks may be required to achieve suitable functional characteristics.
Another example of such a rotor may be referred to as a tangential-magnet rotor configuration, in which the magnetizing direction of the PM bulks is aligned tangentially to the outer perimeter of the rotor core (e.g., perpendicular to a radial axis extending from the axis of rotation through a point on the external perimeter of the rotor). The challenge for this rotor configuration is that there may be insufficient space in the rotor core to install PM bulks due to the limit of the rotor radius. As such, this configuration may require a larger rotor core (e.g., with a larger radius). The larger size can result in additional material requirements and higher costs. Further, in such a configuration, d-axis and q-axis inductances are generally not symmetrical, and complex control configurations may be required to achieve suitable functional characteristics.